A Fjellgangr engaging and engaged by a unit of Bladeslingers uses Thrash but, at the first attack roll, my opponent rolls a double 1.

Question A: the attack must stop immediately and he cannot do rolls for my other models inside MEL range of the Fjellgangr? or he concludes the action attacking other targets and only then he cannot attack again?

Question B: if subsequently in the same round i want to move away my Bladeslingers, the Fjellgangr can try to hit them with Free Strikes? or this count as an attack and so they can exit his MEL range freely since he cannot attack?

A: the attack fails.=he can not make anny more attacks / counter-attacks for the rest of this round

fumbles:

If the dice result of an attack roll consists only of 1’s, then
the attack automatically fails. This counts for melee, missile
and magic attacks but not for spell casting rolls.
A model that fumbled is not allowed to make any further
attacks and counter-attacks for the rest of this round.

B: I would say the model can not make a free strike.
The question is a free stike a free attack? if yes => he can not attack for the rest of the round. (needs a official ruling. I suppose)

Free strike:
A model may perform a free strike at any model that voluntarily leaves or passes through its melee range. A free strike is performed immediately when the model leaves the melee range. After the free strike has been resolved, the attacked model may continue moving and performing actions as normal.
A free-strike is done without spending action tokens and the model performing the free-strike may use any of its attack types. Free-strikes benefit from a +2 to hit and a +2 damage bonus.
Performing free strikes counts as a passive activation.

Corrected

Club: The Green Knight

"You have to learn the rules of the game.
And then you have to play better than anyone else."
Albert Einstein

A: The attack fails and the model is not allowed to make any further attack actions. But since abilities like Trash, Circle of death, ambidextrous fighter etc. make several attacks with one attack action that count as made simultaneously, it CAN still make all attacks of that certain action.

B: Must check in the rulebook (don't have one at hand atm...), but I would say no, they can't make a free strike...